3514
J. Phys. Chem. A 1998, 102, 3514-3523
Excited-State Reaction Dynamics of Chlorine Dioxide in Water from Absolute Resonance Raman Intensities Catherine E. Foster and Philip J. Reid* Department of Chemistry, Box 351700, UniVersity of Washington, Seattle, Washington 98195 ReceiVed: February 4, 1998; In Final Form: March 10, 1998
Resonance Raman spectra of aqueous chlorine dioxide (OClO) are measured for several excitation wavelengths spanning the photochemically relevant 2B1-2A2 optical transition. A mode-specific description of the optically prepared, 2A2 potential energy surface is derived by simultaneous analysis of the absolute resonance Raman and absorption cross sections. The resonance Raman spectra are dominated by transitions involving the symmetric stretch and bend coordinates demonstrating that excited-state structural evolution occurs predominately along these degrees of freedom. Scattering intensity is not observed for transitions involving the asymmetric stretch, demonstrating that excited-state structural evolution along this coordinate is modest. The limited evolution along the asymmetric stretch coordinate results in the preservation of C2V symmetry on the 2A2 surface. It is proposed that this preservation of symmetry is responsible for the increase in Cl photoproduct quantum yield in solution relative to the gas phase. Analysis of the absolute scattering cross sections also demonstrates that the homogeneous line width for the 2B1-2A2 optical transition in water is essentially identical to that in cyclohexane; however, the extent of inhomogeneous broadening increases dramatically in aqueous solution. Comparison of the spectroscopic properties of OClO to the properties of isoelectronic O3- is made to elucidate the origin of the solvent response to OClO photoexcitation. It is suggested that solvent-solute dipole-dipole coupling and intermolecular hydrogen bonding represent the largest components of the solvent coordinate.
Introduction
SCHEME 1
The photochemistry of chlorine dioxide (OClO) is of current interest in atmospheric chemistry due to the participation of this compound in the reactive chlorine reservoir as well as its potential role in stratospheric ozone layer depletion.1-5 An outline of the photochemical processes available to OClO following photoexcitation is presented in Scheme 1. As the scheme illustrates, photoexcitation of OClO results in the formation of either ClO and O or Cl and O2.1,6-49 The Cl and O2 products may be preceded by the formation of the peroxy isomer of chlorine dioxide, ClOO, produced by the photoisomerization of OClO.1,26,27,30,32 ClOO has been observed in low-temperature matrixes; however, its presence in the gas or solution phase has yet to be firmly established.41-43,45,46 The intriguing aspect of this chemistry is that the partitioning between photochemical pathways is dependent on phase. For example, the quantum yield for Cl formation (ΦCl) in the gas phase is ∼0.04 but is reported to be near unity in lowtemperature matrices and on surfaces.9-11,14-16,40,41,44-46 The chemistry in solution is intermediate between these two limits with ΦCl ) 0.1-0.2 in water and methanol.26-28,30,32-34 In addition, the mechanism of Cl formation in solution is sensitive to solvent polarity with the Cl + O2(1∆g) and Cl + O2(3Σg-) channels dominating in nonpolar and polar solvents, respectively.29 Recent solution-phase studies of OClO have attempted to identify the solvent-solute interactions that influence photoproduct formation. Ultrafast transient absorption spectroscopy has been used to monitor the kinetics of ground-state photo* To whom correspondence should be addressed.
product formation.26,33,34 These studies have provided information on the processes that occur following internal conversion to the ground state (geminate recombination, vibrational relaxation, etc.). We have employed resonance Raman intensity analysis (RRIA) in an attempt to understand how the presence of solvent can influence the excited-state reaction dynamics.35,36 Using this technique, a mode-specific description of the excitedstate potential energy surface can be developed, thereby revealing the reaction dynamics that occur immediately following photoexcitation.50-52 By performing RRIA on OClO dissolved in a variety of solvents, environmental dependence of the excited-state reaction dynamics can be quantitatively explored. We performed our first studies in cyclohexane where it was anticipated that the weak intermolecular interactions characteristic of this solvent would allow for direct comparison to the dynamics that occur in the gas phase. In particular, analysis of the high-resolution electronic absorption spectrum of gas-phase OClO has demonstrated that evolution along the asymmetric stretch coordinate is an important component of the excited-state structural relaxation.1,6-8 It was expected that similar dynamics would be observed in cyclohexane; however, we found that the evolution along the asymmetric stretch coordinate is restricted in this solvent.36 We proposed that dipole-induced dipole interactions between OClO and the surrounding solvent were responsible for modification of the optically prepared excited-state surface along the asymmetric
S1089-5639(98)01002-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/28/1998
Reaction Dynamics of Chlorine Dioxide in Water stretch coordinate. The observation that the presence of solvent can influence the excited-state reaction dynamics raises several new questions concerning the condensed-phase reactivity of OClO. What other types of solvent-solute interactions can affect the excited-state dynamics? How do these interactions contribute to the phase- and solvent-dependent photochemical reactivity? Finally, what role does solvent-mediated reactivity play in altering the environmental impact of OClO? In this paper, we report the analysis of the absolute resonance Raman intensities of OClO dissolved in water. This solvent was chosen for a variety of reasons. First, the dominant solvent-solute interactions between OClO and water are expected to be dipole-dipole coupling and intermolecular hydrogen bonding. These two interactions are expected to be stronger than the dipole-induced dipole coupling operative in cyclohexane; therefore, water provides the opportunity to evaluate the influence of strong, solvent-solute interactions on the excited-state reactivity of OClO. Second, the photochemistry of OClO in aqueous environments is of direct relevance to the environmental impact of this compound. RRIA is capable of providing new insight into the excited-state reaction dynamics of OClO in this environmentally important solvent. Third, recent studies of OClO reactivity in water have involved the application of time-resolved optical spectroscopies with interpretation of these studies dependent on the availability of an accurate description of the optically active 2A2 potential energy surface. RRIA provides an elegant method by which to develop this description. The RRIA of aqueous OClO presented here provides a detailed description of the excited-state reaction dynamics. The Raman intensities demonstrate that following photoexcitation the excited-state relaxation dynamics are dominated by evolution along the symmetric stretch and bend coordinates. The absence of intensity for transitions involving the asymmetric stretch demonstrates that excited-state evolution along this coordinate is restricted in comparison to the dynamics that occur in the gas phase. The absence of substantial evolution along the asymmetric stretch results in the preservation of C2V symmetry on the 2A2 surface. This observation in combination with recent ab initio theoretical work suggests that the preservation of C2V symmetry is responsible for the increase in ΦCl in water relative to the gas phase.47-49 The homogeneous line width for the 2B12A transition is found to be 85 ( 15 cm-1. This value is 2 essentially identical to the homogeneous line width determined for OClO dissolved in cyclohexane, suggesting that the energetics and couplings between the excited potential energy surfaces of OClO are not substantially modified by changes in solvent polarity. Finally, a discussion concerning which solvent-solute interactions are operative for aqueous OClO is presented. In particular, comparison is made to recent spectroscopic studies of O3-, a species that is isoelectronic with OClO. The large reduction in line width observed for the symmetric stretch fundamental transition of O3- in D2O relative to H2O is not observed for OClO, demonstrating that the absence of charge results in weaker coupling to the solvent coordinate. Given this observation, it is suggested that for aqueous OClO dipoledipole coupling and potentially intermolecular hydrogen bonding are the largest components of the solvent coordinate. Experimental Section Materials. The absolute Raman intensities of chlorine dioxide (OClO) dissolved in water were measured at 282.4, 319.9, 355, 368.9, 435.7, and 532 nm, using the direct or hydrogen shifted second and third harmonic output of a Nd:
J. Phys. Chem. A, Vol. 102, No. 20, 1998 3515 YAG laser (Spectra-Physics GCL-170, 30 Hz). The synthesis of gaseous OClO was performed as previously described.36,53 Solutions of OClO in water were prepared by bubbling gaseous OClO through an aqueous solution of 1 M KNO3. Adjustment of the OClO concentration was performed by diluting the concentrated aqueous OClO solution with additional amounts of aqueous 1 M KNO3 to produce the samples employed here. Quantitative addition of KNO3 was performed such that the NO3- transition at 1049 cm-1 could be used as an internal standard for determining absolute scattering cross sections (see below).54,55 Resonance Raman Spectra. Resonance Raman spectra were obtained by flowing the aqueous OClO sample through a wire guided jet (282.4, 319.9, 355, and 368.9 nm) or square walled glass capillary (355, 368.9, 436, and 532 nm). The sample was delivered at a rate sufficient to replenish the illuminated volume between excitation events. The 135° backscattering geometry was employed in all experiments. The scattered light was collected using standard, UV-quality refractive optics and delivered to a 0.5 or 0.75 m single-stage spectrograph (Acton Research Corp.) equipped with either a classically ruled 1200 grooves/mm or a holographic 2400 grooves/mm grating and a polarization scrambler placed at the spectrograph entrance. Raman scattering was detected with a liquid N2 cooled 1100 × 330 pixel back-illuminated CCD detector (Princeton Instruments). The intensity of OClO scattering was found to be linear with pump power up to 25 µJ/pulse at 368.9 nm; therefore, 1000:1. The polarization of the scattered light was defined using a large-aperture calcite polarizer placed before the polarization scrambler in front of the spectrograph entrance.
3516 J. Phys. Chem. A, Vol. 102, No. 20, 1998
Foster and Reid
TABLE 1: Absolute Raman Cross Sections for Chlorine Dioxide in Water excitation energy (×10-3 cm-1)
ν1 (×1010 Å2)a
2ν1 (×1010 Å2)
18.8 22.9 27.1 28.2 31.3 35.4
0.06 ( 0.02c 1.7 ( 0.3 8.0 ( 1.6 11.0 ( 1.8 5.9 ( 1.5 1.9 ( 0.4
0.7 ( 0.2 5.1 ( 1.2 9.1 ( 2.0 5.1 ( 1.3 0.95 ( 0.02
ν2 (×1010 Å2)
1.7 ( 0.3
2ν2 (×1010 Å2)
0.6 ( 0.2
2ν3 (×1010 Å2)
F(ν1)b 0.14 ( 0.01 0.24 ( 0.03 0.28 ( 0.01 0.29 ( 0.07 0.22 ( 0.02 0.18 ( 0.04
